Pyruvate Kinase (PK) converts phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP) to pyruvate and adenosine triphosphate (ATP), respectively, which is the final step in glycolysis. In humans, four PK isoforms are expressed by two structural genes. The PKLR gene encodes PKR and PKL tissue specific isoforms expressed in erythroid cells and liver, respectively.
Pyruvate kinase R (PKR) is the isoform of pyruvate kinase expressed in red blood cells (RBC) and is a key enzyme in glycolysis. Activation of PKR is proposed to directly target both sickling by reducing deoxy-HgbS and hemolysis by improving RBC membrane integrity. Specifically, PKR activation inhibits Hgb deoxygenation and sickling by decreasing levels of 2,3-diphosphoglycerate (2,3-DPG) and increasing oxygen affinity of HgbS. Furthermore, PKR activation increases adenosine triphosphate (ATP), which has been shown to support overall RBC membrane integrity and stress resilience, thus potentially decreasing hemolysis. ATP also supports elimination of reactive oxygen species (ROS) which damage RBC and impair their functionality, and reduces vascular adhesion associated with membrane injuries.
Sickle cell disease (SCD) is a chronic hemolytic anemia caused by inheritance of a mutated form of hemoglobin (Hgb), sickle Hgb (HgbS). It is the most common inherited hemolytic anemia, affecting 70,000 to 80,000 patients in the United States (US). SCD is characterized by polymerization of Hgb S in red blood cells (RBCs) when HgbS is in the deoxygenated state (deoxy-HgbS), resulting in a sickle-shaped deformation. Sickled cells aggregate in capillaries precipitating vaso-occlusive events that generally present as acute and painful crises resulting in tissue ischemia, infarction, and long-term tissue damage. RBCs in patients with SCD tend to be fragile due to sickling and other factors, and the mechanical trauma of circulation causes hemolysis and chronic anemia. Finally, damaged RBCs have abnormal surfaces that adhere to and damage vascular endothelium, provoking a proliferative/inflammatory response that underlies large-vessel stroke and potentially pulmonary-artery hypertension. Collectively, these contribute to the significant morbidity and increased mortality associated with this disease.
Currently, therapeutic treatment of SCD is inadequate. The only known cure for SCD is hematopoietic stem cell transplantation which has serious risks, is typically recommended for only the most serious cases, and is largely offered only to children with sibling-matched donors. Gene therapy is also under investigation with promising preliminary results; however, there are market access hurdles, mainly high cost and treatment complexities, that are likely to limit its broad use in the near term. There have been few advances in therapies for SCD over the past two decades. Hydroxyurea (HU) induces HgbF which interrupts the polymerization of HgbS, and thereby has activity in decreasing the onset of vaso-occlusive crises and pathological sequelae of SCD. While HU is in wide use as a backbone therapy for SCD, it remains only partially effective, and is associated with toxicity, such as myelosuppression and teratogenicity. Patients receiving HU still experience hemolysis, anemia, and vaso-occlusive crises, suggesting a need for more effective therapies, either as a replacement or in combination with HU. Beyond HU, therapeutic intervention is largely supportive care, aimed at managing the symptoms of SCD. For instance, blood transfusions help with the anemia and other SCD complications by increasing the number of normal RBCs. However, repeated transfusions lead to iron overload and the need for chelation therapies to avoid consequent tissue damage. In addition to these approaches, analgesic medications are used to manage pain.
Given the current standard of care for SCD, there is a clear medical need for a noninvasive, disease-modifying therapy with appropriate safety and efficacy profiles.